The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
The present commercial wireless communications systems employ radio frequencies for communications between base stations and mobile phones. Currently the fourth generation (4G) of the wireless communications systems are increasingly deployed and used. The 4G systems provide mobile ultra-broadband Internet access, for example to laptops with Universal Serial Bus (USB) wireless modems, to smartphones, and to other mobile devices.
The International Telecommunications Union-Radio communications sector (ITU-R) have specified a set of requirements for 4G wireless communications standards, named the International Mobile Telecommunications Advanced (IMT-Advanced) specification. The IMT-Advanced defines peak speed requirements for a 4G service at 100 megabits per second (Mbit/s) for high mobility communication (such as from trains and cars) and 1 gigabit per second (Gbit/s) for low mobility communication (such as pedestrians and stationary users).
The first-release versions of Mobile Worldwide Interoperability for Microwave Acces (WiMAX) and the Long Term Evolution (LTE) defined by the Third Generation Partnership Project (3GPP) support much less than 1 Gbit/s peak bit rate, they are not fully IMT-Advanced compliant, but are often branded 4G by service providers.
Beyond 4G (B4G) radio systems are currently being developed. These systems are envisaged to be commercially available in the following decades. The Orthogonal Frequency Division Multiple Access (OFDMA) is the strongest candidate for the access method in the B4G system.
It is foreseen that the cell sizes of the B4G systems are smaller and have significantly higher spectrum efficiency than the present systems. It would be desirable to increase the spectrum efficiency especially in terms of increasing the efficiency of the net information transferred per a unit of spectrum.
When error correction coding, for example Forward Error Correction coding (FEC), is used in wireless transmissions, spectral efficiency can be increased by using a higher coding rate, i.e. a proportion of net information, k, per gross information, n, generated in the coding. The coding rate is conventionally denoted as the ratio of the net and gross information, k/n, e.g. 3 bits/4 bits. The spectral efficiency can be measured in bits/s/Hz.
Different modulation schemes may be used to modulate a radio frequency carrier. The used modulation scheme defines a number of bits to be used per each modulation symbol. Accordingly, the selection of the modulation scheme can also be made such that the spectral efficiency is optimized.
Various modulation and error correction coding schemes may be defined into a set of combinations of Modulation and Coding Schemes (MCSs), such as in the LTE systems, where MCSs are defined for example in 3GPP TS 36.213 Physical Layer Procedures v 10.8.0 Chapter 7.1.7.1.
In LTE, subcarriers that are allocated as transmission resources to User Equipment (UE) in a certain transmission time instant use the same MCS and transmission power. On the other hand in downlink, the transmission power for different subcarriers is determined by the base station, i.e. eNodeB in LTE networks. Accordingly, the different characteristics of the radio channels of the subcarriers are not considered.
On the other hand, with dense deployment needed for the high spectrum efficiency requirement of the B4G systems, the number of UE is roughly the same as the number of cells of the B4G system. Accordingly, a typical number of allocated frequency resources for a single UE in a B4G system could be high and close to the maximum system bandwidth.
Resource allocation with wide frequency bandwidth means that the channel likely fades within the allocated resources. Since OFDMA has no built-in diversity, its performance is very dependent on the coding rate. When a high coding rate is employed for the allocated resources, OFDMA performs poorly because coding does not manage to compensate the influence of weak subcarriers. Then, one deep fade or strong interference can result in that the whole packet needs to be re-transmitted. Therefore, in order to maintain OFDMA performance in the B4G system, the MCS of a B4G system should be selected according to the worst sub-carries on the bandwidth. However, this would not facilitate maintaining high spectrum efficiency.